style: Add simple parsing and matching support for :has

Parsing is behind a config value `layout.css.has-selectors.enabled`. This
change does not support p:has(> a) combinators, but will handle them
gracefully, just not matching on them.

Differential Revision: https://phabricator.services.mozilla.com/D149515

components/style/invalidation/element/element_wrapper.rs

@@ -285,6 +285,11 @@ where
         Some(Self::new(sibling, self.snapshot_map))
     }
 
+    fn first_element_child(&self) -> Option<Self> {
+        let child = self.element.first_element_child()?;
+        Some(Self::new(child, self.snapshot_map))
+    }
+
     #[inline]
     fn is_html_element_in_html_document(&self) -> bool {
         self.element.is_html_element_in_html_document()